To adhere the solid particles, different types of binders have been used. Such binders can be of either organic or inorganic nature. Organic binders are usually phenol-based and to a large extent preferred by iron and metal foundries. However, phenol-based binders are detrimental to the environment as during the casting, the heating of phenol-based binders entails pyrolysis whereby CO and polyaromatic hydrocarbons (PAH) are released, among others. Furthermore, the casting leaves some sand which is polluted with toxic organic compounds. The European iron and metal foundries alone produce 7,000,000 tons of polluted sand every year.
Consequently, extensive research has been performed to provide inorganic binders. The use of water glass as a binder has been suggested but so far, the use of water glass as a binder has not been widespread within the iron and metal casting industry.
Water glass as a binder was used to adhere sand in foundries for the first time about 1920. In this process, sand is mixed with an aqueous solution of water glass, and this mixture is subsequently filled into a mould to be cured by evaporation of the water. This process was essentially improved about 1950 by the development of the CO.sub.2 process in which the CO.sub.2 gas is led through the wet mixture of sand and water glass in order to cure the binder. Cores and moulds obtained by the CO.sub.2 process are relatively porous and their strength is weak.
At the end of the 1960s, self-curing binding systems were developed. The curing is effected by supplying an organic ester to the wet mixture of sand and water glass before filling it into a mould.
At the end of the 1970s, there was developed a dry curing process where energy from warm air, a warm mould, or microwaves entail curing of the water glass binder.
In the early nineties, the present inventor developed a curing process where warm air was blown into a ventilated mould by use of a three-step temperature cycle, cf. "Milj.o slashed.projekt nr. 189, Milj.o slashed.styrelsen".
All the aforesaid processes in which water glass is used as a binder have, however, the inconvenience that the flowing properties of the mixture of sand, water glass, and water are relatively poor when filling the mould. This entails that the mixture tends not to flow into all the cavities of the mould. The known processes using water glass are thus limited to be used in connection with moulds showing no detailed surface.